The invention relates to an insulated container for storing liquid helium.
Such containers for storing liquid helium have a neck-shaped opening for drawing off the liquid. For this purpose there is introduced into the container a draw-off siphon which reaches to the bottom of the container and which consists essentially of a vacuum-insulated tubular line having a vacuum closure and safety valve arranged at its top end. A filling tube is connected via a rigid or flexible intermediate piece to the draw-off siphon. In many cases the draw-off siphon is also provided with a shut-off valve.
At a boiling temperature of helium of approximately -269.degree. C. a raised evaporation rate occurs during operation of the draw-off siphon, because the latter constitutes a temperature bridge to the atmosphere. According to U.S. Pat. No. 3,302,419 this rate should be lowered by cooling the draw-off siphon with evaporated medium. During stoppage times the container nevertheless still has a raised evaporation rate because of heat conduction to the liquid helium via the siphon tube. Cooling with evaporated medium gives no decisive benefit in this respect, because with modern helium containers having waste gas cooling the waste gas energy is already used up for cooling the insulation. For this reason it has hitherto been customary to store the draw-off siphon for helium containers outside the container and to insert the draw-off siphon into the container only when it is intended to draw off liquid helium.
This operation is dangerous (risk of combustion) because of the outflow of cold medium, and requires heavy expenditure for personnel and an appropriately high room, since the draw-off siphon has a height almost equal to that of the container and has to be inserted at container height into the neck-shaped opening of the container, or must be assembled from a plurality of parts screwed to one another for its insertion.
Whenever the draw-off siphon heated to ambient temperature is introduced, helium is evaporated.